1. Field of the Invention
The present invention relates to a temperature sensor integrated on a semiconductor substrate.
2. Description of the Related Art
As the conventional temperature sensors integrated on a semiconductor substrate, there is a temperature sensor shown in FIGS. 1A, 1B, 2A and 2B. FIG. 1A is a sectional view showing a part of a configuration of a conventional temperature sensor. FIG. 1B is a plan view corresponding to the sectional view in FIG. 1A. FIG. 2A is a characteristic view showing temperature dependence of potential at respective portions of the circuit. FIG. 2B is a characteristic view showing dispersion or scatter of detected temperatures.
First, a configuration of the temperature sensor will be explained with reference to FIG. 1A hereunder. A circuit shown in FIG. 1A is composed of an n.sup.+ diffusion region 2 and a p.sup.+ diffusion region 3 formed in a p type substrate 1, an interlayer insulating layer 4, a polysilicon resistor layer 5, an Al wiring layer 6, and a potential level discriminator 7. The potential level discriminator 7 is made up of resistors R8, R9 and a comparator 10. An explanation will be made in the following under the assumption that an input impedance of the comparator 10 is infinity. One end of the polysilicon resistor layer 5 is connected to VDD, and the p type substrate 1 is connected to GND via the p.sup.+ diffusion region 3. Needless to say, though not shown as a device structure in FIG. 1A, the potential level discriminator 7 may be integrated on the p type substrate 1. The resistors R8, R9 in the potential level discriminator 7 are formed of polysilicon resistor which can be formed by the same process as the manufacturing process of the device whose sectional structure is shown in FIG. 1A, etc. In the structure in FIG. 1A, a p-n junction between the n.sup.+ diffusion region 2 and the p type substrate 1 serving as GND is isolated by reverse-biasing. A leakage current flows from the n.sup.+ diffusion region 2 to the p type substrate 1 via the p-n junction interface, i.e., there exists a leakage path via the junction.
Next, an operation of the temperature sensor will be explained with reference to FIG. 2A hereunder. In FIG. 2A, an ordinate indicates potential at respective portions of the circuit and an abscissa indicates a temperature of the semiconductor substrate in which this circuit is merged. At first, behavior of potential at a point A in FIG. 1A will be explained. Though the value of the leakage current flowing from the n.sup.+ diffusion region 2 to the p type substrate 1 is extremely small at near room temperature, such leakage current is increased like an exponential function when the temperature of the semiconductor substrate, i.e., the p type substrate 1 is increased. Hence, when the temperature of the semiconductor substrate is increased higher, potential at the point A becomes lower due to an increase in the leakage current flowing from the n.sup.+ diffusion region 2 to the p type substrate 1. The value of the leakage current can be represented by a variable equivalent resistance (leakage conductance) from the n.sup.+ diffusion region 2 to the p type substrate 1. Such variable leakage conductance is in proportion to an area of a junction isolation surface, i.e., an area of the n+ diffusion region 2. Meanwhile, potential drop at the point A can be determined by a resistance dividing ratio of the resistance value of the polysilicon resistance layer 5 to the variable equivalent resistance. Accordingly, potential drop at the point A can be determined by both the area of the n.sup.+ diffusion layer 2 and the resistance value of the polysilicon resistance layer 5.
Next, care to potential at a point B in the potential level discriminator 7 will be taken hereunder. Assume that VDD supplied to this circuit has no dependence upon the semiconductor substrate temperature, potential at the point B does not have dependence upon the semiconductor substrate temperature so that the value can be decided by a resistance dividing ratio between the resistor R8 and the resistor R9. Hence, if potential at the point B is set low rather than that at the point A when the semiconductor substrate temperature is low, potential at the point A is gradually lowered as the semiconductor substrate temperature is increased. Thus, a point at which potential at the point A intersects with potential at the point B can be obtained at a certain temperature. Therefore, if such intersect point is assumed as a detected temperature Tx and then this point is detected by the comparator, it can be detected when the semiconductor substrate temperature comes up to the detected temperature Tx. At this time, an output of the comparator is shifted from "L (low level)" to "H (high level)".
However, there has been problems set forth in the following in such related art.
First, there has been such a problem that scatter of the detected temperature Tx generated by scatter in manufacturing process is considerable. The major scatter in manufacturing process is the scatter of the leakage current generated by the scatter of impurity concentration in the region which forms the leakage path. Hence, the scatter of the detected temperatures Tx occurs due to the scatter in the leakage current in a relationship between the potential at the point A and the semiconductor substrate temperature. As shown in FIG. 2B, a considerable scatter .DELTA.Tx is generated in the actually detected temperature Tx. Further, it has been known that generally such scatter of the leakage current is relatively large and, of course, the scatter .DELTA.Tx of the detected temperature is also significant. This aspect will be explained later in detail in embodiments with reference to the results of rough estimation.
Also, as evident from the plan view shown in FIG. 1B, since the n.sup.+ diffusion region 2 is brought into contact with the Al wiring pattern 6 via only one window, i.e., a contact hole 21, the temperature sensor is susceptible to scatter of resistance in the n.sup.+ diffusion region caused by scatter in manufacturing process.
It will be explained in the following why several serious problems are caused if the scatter .DELTA.Tx is included in the detected temperature.
The first problem is a disadvantage caused when an upper limit of the scatter .DELTA.Tx of the detected temperature exceeds an upper limit temperature of the package and the bonding wire, or conversely a lower limit of the scatter .DELTA.Tx of the detected temperature is lowered to enter into the normal operation temperature range of the circuit main body. First, if the upper limit of the scatter .DELTA.Tx of the detected temperature exceeds the upper limit temperature of the package and the bonding wire, troubles in the reliability of the package and the bonding wire are caused before an output indicating the fact that the temperature of the semiconductor circuit has been increased can be output by the temperature sensor, so that it is likely that functions of IC are lost. While, in the IC chip in which the temperature sensor is merged, it is common to halt or limit an operation of the circuit main body using the output of the temperature sensor. If the lower limit of the scatter .DELTA.Tx of the detected temperature is within the normal operation temperature range of the circuit main body, such a situation occurs that the operation of the circuit main body is halted or limited though the circuit is to be normally operated at the temperature. As a result, the detected temperature of the temperature sensor should be set to be lower than the upper limit temperature of the package or the bonding wire but higher than the normal operation temperature range of the circuit main body. In other words, the scatter .DELTA.Tx of the detected temperature must be suppressed within an extremely small scattering range.
The second problem is that, since the n.sup.+ diffusion region 2 is formed as a rectangle as shown in the plan view in FIG. 1B, electric field concentration occurs at four corners of the n.sup.+ diffusion region 2, so that a distribution of the leakage current value is deviated. Therefore the conventional temperature sensor is not preferable in reliability.
The third problem is that a chip area for the temperature sensor becomes larger. An important aspect of the circuit is that, normally, the temperature from which the potential of A point descending should be set within a temperature range (150 to 200.degree. C.), which is required as the detected temperature range of the temperature sensor. By the way, in this related art, the n.sup.+ diffusion region to determine the leakage current value has been assumed to have a popular area (50 .mu.m.times.50 .mu.m) and the polysilicon resistance necessary for the detection at 175.degree. C. has been calculated by the so-called SPICE simulator (simulation equipment for analogue circuits) under the following conditions. That is, in this simulation, it has been assumed that VDD=5 V, and the potential at which a curve representing the characteristic of the A point intersects that of the B point in FIG. 2A is 2.5 V. Further, the leakage current of the n.sup.+ diffusion region has been modeled by the p-n diode, and the leakage current value has been calculated with considering SPICE parameters (area parameter only, others are in default) in the situation that the leakage path is formed of the 50 .mu.m.times.50 .mu.m p-n junction and the leakage current at 180.degree. C. is set to 0.6 .mu.A with reference to actually measured data. As a result, the polysilicon resistance value of 5.3 M.OMEGA. has been needed.
A very long polysilicon resistor pattern is needed to achieve the polysilicon resistance value of 5.3 M.OMEGA. Hence, in the layout in which the polysilicon resistor pattern 5 is formed on the n.sup.+ diffusion region 2, as shown in FIG. 1A, an area necessary for the polysilicon resistor pattern 5 is increased.
Like the above, in the conventional temperature sensor, there have been the problem caused by scatter of the detected temperature and the problem that an area necessary for formation of the circuit is increased.
In addition, it may be thought of that a hysteresis circuit is attached to stabilize the detected temperature output in the neighborhood of the detected temperature. However, if the hysteresis circuit is attached to the conventional temperature sensor, there has been the problem that scatters of the detected temperature and the return temperature would be increased due to scattering in manufacturing process, which is generated by an influence of the leakage path, rather than the case where no hysteresis circuit is attached.